Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Qual conferência se juntou em 1954 com um mascote de castores?
 ###Tabelas: CREATE TABLE minesweepers (minesweepers_id AUTO_INCREMENT PRIMARY KEY, Class VARCHAR, Ships in class VARCHAR, Pennant Number INT, Builder VARCHAR, Commissioned VARCHAR) CREATE TABLE official_versions (official_versions_id AUTO_INCREMENT PRIMARY KEY, Version VARCHAR, Length VARCHAR, Album VARCHAR, Remixed by VARCHAR, Year INT) CREATE TABLE old_northern_state_conference_membership (old_northern_state_conference_membership_id AUTO_INCREMENT PRIMARY KEY, School (IHSAA ID#) VARCHAR, Location VARCHAR, Mascot VARCHAR, # / County VARCHAR, Year Joined INT, Year Left INT, Conference Joined VARCHAR) CREATE TABLE conference_standings (conference_standings_id AUTO_INCREMENT PRIMARY KEY, Team VARCHAR, SEC Wins INT, SEC Losses INT, Percentage VARCHAR, Home Record VARCHAR, Road Record VARCHAR, Overall Record VARCHAR) CREATE TABLE prize_money_won (prize_money_won_id AUTO_INCREMENT PRIMARY KEY, Player VARCHAR, Group Legs won INT, Play-Off Legs won INT, Winners Group Legs won INT, Winners Play-Off Legs won INT, Final position money won* VARCHAR, Total money won VARCHAR)  
SELECT Conference Joined FROM old_northern_state_conference_membership WHERE Year Joined = 1954 AND Mascot = beavers